A switched mode power supply (SMPS) is an electronic power supply that incorporates switching and energy storage elements so as to achieve efficient conversion of electrical power from a power source to a load. A SMPS may convert DC-DC, AC-DC or DC-AC. There are three basic configurations or topologies of dual switch, single storage element SMPS; Buck, Boost and Buck-Boost as illustrated in FIG. 1. In each case, the power delivered to the load 1 is controlled by the duty cycle of the control signal applied to the control terminal of the active switching elements (i.e. the gate of the MOSFET 4 in FIG. 1(b)). In each case, a first (input) switch 4 when switched on, transfers energy from an input source 2 to an energy storage device 3 (e.g. an inductor). The first switch 4 may be a MOSFET or similar semiconductor switching device. When the first switch 4 is turned off, a second switch 5 is employed to transfer the energy stored in the inductor to the load 1. In its simplest form, the second switch 5 is a diode or similar semiconductor device. It will be appreciated that in the exemplary circuits illustrated in FIG. 1(b) a capacitor is shown as part of the load however this is for convenience of explanation and generally the capacitor is part of the SMPS.
Broadly speaking a SMPS operates so as to transfer energy from an input power source to an output load via an energy storage element. This is achieved through the operation of the switching elements so that during the first portion of the switching cycle, energy is transferred from the input source to the energy storage element and during the second portion of the switching cycle energy is transferred from the energy storage element to the output load. The ratio of the first portion of the switching cycle to the total switching cycle is referred to as the duty cycle of the SMPS. The power delivered to the load is controlled by the duty cycle.
A SMPS may operate in one of three different modes namely: DCM (Discontinuous Conduction Mode), BCM (Boundary Conduction Mode) and CCM (Continuous Conduction Mode). In DCM, the energy storage element is reset before the end of each switching cycle. In CCM, the energy storage element is storing some energy (non zero value) at the end/start of each switching cycle. In BCM, the energy storage element is emptied (reset to zero) precisely at the end of each switching cycle. i.e. the SMPS is operating at the boundary between DCM and CCM.
An SMPS controller is a device whose purpose is to control the output quantity (typically voltage or current) delivered by the SMPS to the load by observing various quantities (typically voltages or currents) within the SMPS which may or may not include the output quantities themselves and adjusting the on and off times of one or more switches within the SMPS according to the desired mode of operation (CCM, BCM or DCM) of the SMPS.
As well as providing control for the SMPS, the controller itself might also implement other ancillary functions such as, but not limited to: controlled start-up, fault protection (over voltage, over current, over temperature), standby and sleep modes as well as any other functionality that may be required for the specific application in which the power supply is being used.
An offline AC-DC SMPS is an electronic power supply that converts an incoming AC supply voltage into a DC output. An AC-DC SMPS typically incorporates a multiplicity of switching and energy storage elements configured so as to achieve efficient conversion of electrical power from the AC mains voltage supply to a load. Generally, the AC voltage is first converted by means of a rectifier or similar circuit to a rectified form. The rectified AC voltage is used as the input to the switching stage of the SMPS. The rectifier very often is a diode bridge rectifier. A bulk capacitor is typically used to smooth the rectifier output before the SMPS. If the bulk capacitor is sufficiently large, the input to the SMPS is a pseudo-DC voltage with a ripple voltage present at twice the mains frequency. The amount of ripple depends both on the size of the bulk capacitor and the power drawn by the SMPS. In some cases it is desirable to have a small value of bulk capacitance at the output of the bridge rectifier so that the fully or near fully rectified AC mains voltage appears at the input to the SMPS.
FIG. 2 below outlines a typical arrangement for an offline AC-DC SMPS which includes a diode bridge rectifier 10, and a bulk capacitor 12 for smoothing the rectified mains. The rectified mains is provided as an input source voltage to the SMPS 14 which in turn converts it for delivery to a load 1.
In offline AC-DC SMPS converter applications, for safety reasons it is often required to have isolation between the AC mains and the load of the SMPS converter. This may be achieved in an SMPS using a transformer as the energy storage device. In lower power applications, one of the most utilised isolated SMPS converter topologies is the Flyback converter, shown in FIG. 3 which is essentially an isolated version of the Buck-Boost topology. A flyback transformer 16 achieves the required isolation between the primary and secondary sides; the transformer 16 also allows for voltage scaling by appropriate selection of the turns ratio of the transformer, which is the ratio between the number of turns on the primary winding and the number of turns on the secondary winding (Np:Ns).
Additional circuitry may be included within an offline AC-DC SMPS converter for purposes other than that of actual power conversion. Thus for example, in the exemplary arrangement of FIG. 4, an EMI filter (shown in block form with protection circuitry 20) is provided to limit the transfer of switching noise onto the mains 22. Similarly, protection circuits or devices may be provided, for example, to protect against over-voltage (surge protection) and over-current protection. Typically, one or either or both the EMI filter and protection devices are placed at the front end before the diode bridge rectifier 10 in order to meet the various regulatory compliance standards required when interfacing a circuit to the AC mains supply.
As with the general offline SMPS converters described above offline SMPS converters generally include an EMI filter and some form of over-voltage/over-current protection placed at the front end, followed by a diode bridge rectifier and bulk capacitor after which is some form of SMPS converter (typically isolated), controlled so as to deliver a constant current or voltage to the load which may also have some form of filtering across it (possibly in the form of one or multiple capacitors in parallel with the load).
One difficulty with offline SMPS control is that since isolation is generally required, the observation and feedback of secondary side quantities to the primary side are more complex due to the requirement to maintain the isolation barrier. One solution is to employ an isolated feedback device such as either an optical, capacitive or inductive based couplers circuits to provide isolated feedback to the controller of measurements from the secondary side, for example output voltage, output current or both. The use of isolated feedback devices is not ideal. For example, opto-couplers are a known weakness in isolated SMPS systems as they age badly; especially at higher temperatures, thus leading to a degradation of performance and accuracy over time as well as reducing the useful lifetime of the system. They also complicate the system stability due to the addition of extra poles and zeros in the overall control loop of the SMPS which make designing systems that use opto-coupler circuits more complicated, more costly and physically larger. Similarly, inductive coupling may be used in place of opto-couplers and whilst more reliable and accurate are costly in terms of area/volume and can also suffer significantly from interference.
Accordingly, to avoid the need for isolated feedback devices and other reasons, some switched mode power supply configurations have emerged that use what is termed primary side regulation. In primary side regulation, only quantities that are available on the primary side are measured. From these measurements, an inference or estimate of the output quantities is made. Primary side regulation removes the requirement for an opto-coupler to feed back secondary side quantities across the isolation barrier to the primary side while still maintaining the galvanic isolation. A disadvantage of these systems is that they may require complex manipulations of the primary side quantities (resulting in a commensurately complex implementation) and/or rely on a specific mode of operation (DCM) to work properly as they generally require that the transformer is reset on each cycle (de-energised). Moreover, they can be inaccurate in estimating the output quantities due to the complexity of the calculations required leading to a wide variability in performance between individual realisations of the same implementation.
Primary side control schemes take measurements on the primary side. Typically, they use an auxiliary winding on the transformer as a sensor to indicate the state of the transformer. Using the measurements from the auxiliary winding with those from the primary side, one is able to effectively infer or estimate the output quantities, thus removing the requirement for an opto-coupler to relay the secondary side feedback quantities across the isolation barrier to the primary side, while still maintaining the galvanic isolation. For isolated current drivers it is desirable to accurately control the output current. These systems may require complex manipulations of the primary side quantities and can be inaccurate in estimating the output quantities due to the complexity of the calculations required leading to a wide variability in performance or may rely on a specific mode of operation (DCM) to work (which can compromise the design in terms of overall performance metrics such as cost, reliability, efficiency and EMI)
The use of a peak control is effective but requires that the circuit be operating in DCM, i.e. that the energy in the transformer is completely discharged, as otherwise residual energy can introduce errors. There is therefore a need for a control scheme that permits regulation of the output quantities through primary side control that is not restricted to DCM and does not rely on complex manipulations of the primary side values.
The present application addresses this and other problems.